1. Field of the Invention
This invention relates to a microwave antenna. More particularly, the invention provides a flat panel array antenna utilizing cavity coupling to simplify corporate feed network requirements.
2. Description of Related Art
Flat panel array antenna technology has not been extensively applied within the licensed commercial microwave point to point or point to multipoint market, where more stringent electromagnetic radiation envelope characteristics consistent with efficient spectrum management are common. Antenna solutions derived from traditional reflector antenna configurations such as prime focus fed axi-symmetric geometries provide high levels of antenna directivity and gain at relatively low cost. However, the extensive structure of a reflector dish and associated feed may require significantly enhanced support structure to withstand wind loads, which may increase overall costs. Further, the increased size of reflector antenna assemblies and the support structure required may be viewed as a visual blight.
Array antennas typically utilize either printed circuit technology or waveguide technology. The components of the array which interface with free-space, known as the elements, typically utilize microstrip geometries, such as patches, dipoles or slots, or waveguide components such as horns, or slots respectively. The various elements are interconnected by a feed network, so that the resulting electromagnetic radiation characteristics of the antenna conform to desired characteristics, such as the antenna beam pointing direction, directivity, and sidelobe distribution.
Flat panel arrays may be formed, for example, using waveguide or printed slot arrays in either resonant or travelling wave configurations. Resonant configurations typically cannot achieve the requisite electromagnetic characteristics over the bandwidths utilized in the terrestrial point-to-point market sector, whilst travelling wave arrays typically provide a mainbeam radiation pattern which moves in angular position with frequency. Because terrestrial point to point communications generally operate with Go/Return channels spaced over different parts of the frequency band being utilized, movement of the mainbeam with respect to frequency may prevent simultaneous efficient alignment of the link for both channels.
Corporate fed waveguide or slot elements may enable fixed beam antennas exhibiting suitable characteristics. However, it may be necessary to select an element spacing which is generally less than one wavelength, in order to avoid the generation of secondary beams known as grating lobes, which do not respect regulatory requirements, and detract from the antenna efficiency. This close element spacing may conflict with the feed network dimensions. For example, in order to accommodate impedance matching and/or phase equalization, a larger element spacing is required to provide sufficient volume to accommodate not only the feed network, but also sufficient material for electrical and mechanical wall contact between adjacent transmission lines (thereby isolating adjacent lines and preventing unwanted interline coupling/cross-talk).
The elements of antenna arrays may be characterized by the array dimensions, such as a 2N×2M element array where N and M are integers. In a typical N×M corporate fed array, (N×M)−1 T-type power dividers may be required, along with N×M feed bends and multiple N×M stepped transitions in order to provide acceptable VSWR performance. Thereby, the feed network requirements may be a limiting factor of space efficient corporate fed flat panel arrays.
Therefore it is an object of the invention to provide an apparatus that overcomes limitations in the prior art, and in so doing present a solution that allows such a flat panel antenna to provide electrical performance approaching that of much larger traditional reflector antennas which meet the most stringent electrical specifications over the operating band used for a typical microwave communication link.